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7.0 GROUND WATER--Continued 7.3 Water Levels in Wells Water Levels in Wells Are Determined by Aquifer Properties and Indicate the Direction of Ground-Wafer Flow The degree of aquifer confinement and the ability of an aquifer to transmit watera((ect waterlevels in wells. The difference in elevation between the water level and the producing horizon in a well indicates the degree of aquifer confinement. If an aquifer is confined between less permeable layers, ground wa- ter is under artesian pressure. The level to which water rises in a well penetrating a confined aquifer indicates the amount of artesian pressure in the aquifer. The artesian pressure in the Trinidad Sand- stone increases from well A to wells B and C as the thickness of less permeable rocks overlying the Trinidad Sandstone increases (fig. 7.3-I). In Huer- fano County, Colo., attd Colfax County, N. Mcx., water flows from wells drilled into the Dakota Sand- stone through several hundred feet of impermeable Cretaceous shale (Griggs, 1948, p. 53; McLaughlin, 1966, p. 65). In unconfined aquifers--that is, aqui- fers not overlain by less permeable formations, water is under atmospheric pressure, and water levels in wells coincide with the top oC the zone of saturation (well D, fig. 7.3-1). Area-wide contouring of water levels in an aqui- fer in any year produces a map that indicates the direction of regional ground-water flow. The region- al Clow in Area 61 is from west to east (fig. 7.3-2). However, little ground water discharges on the east side of the area because much of the water itr the system is intercepted by stream valleys. In these valleys, water flowing through permeable layers and along fractures and faults discharges as springs where water-yielding rocks are exposed or flows into alluvi- um where [he alluvium overlies the rocks. Deflec- tions of ground-water level contours around stream valleys in figure 7.3-2 are caused by these local (low systems. Water-level fluctuations in wells from year to year show the response of an aquifer to seasonal and annual variations in the availability of water. Graphs of ground-water fluctuations in wells penetrating the Cuchara-Poison Canyon aquifer, Raton-Vermejo- Trinidad aquifer, and alluvium are shown in figure 7.3-3. The well in the Cuchara-Poison Canyon aqui- fer is 143 feet deep, the well in the Raton-Vermejo- Trinidad aquifer is 62 feet deep, and the well in the alluvium is 26 feet deep. The Cuchara-Poison Can- yon aquifer is confined--the other aquifers, uncon- fined. Annual fluctuations in water levels are great- est in the shallow, unconfined aquifers because these rocks receive water primarily by downward drainage from the surface and are, thus, affected more by precipitation cycles. These wells may go dry during prolonged drought. Because the recharge of water to confined aquifers is controlled primarily by the thick- ness of the overlying confining layer and the ability of the aquifer and confining layer to transmit water, water levels in wells penetrating deep confined aqui- fers respond less to precipitation cycles and usually Iluctuate less than in unconfined aquifers. Within confined aquifers, the wafer-level fluc- tuation in wells Crom year to year depends on varia- tions in the pumping rate and the ability of the aquifer to transmit water. Water-level changes in the Raton-Vermejo-Trinidad aquifer near Trinidad, Colo., ranged from +5 to -23 feet from 1978 to 1981 (fig. 7.3-2).